Tanks are used to store a wide variety of fluids and gases, especially petroleum products, as well as other liquids containing or consisting of various chemicals, and also for storing water. Many of these tanks, or the piping connected to them, when tested, have been found to be leaking. Some current estimates indicate that from 1 to 5% or even more of such tanks leak. Leaking tanks or leaking piping connected to them contaminate the ground water and cause other types of environmental pollution and also become health and safety hazards.
In recent years there has been much concern because the tank testing methods heretofore used to measure the amount of leakage from such tanks have been seriously inaccurate. The inaccuracies have been due to many measurement errors traceable to the equipment used, to significant limitations of the actual testing devices, to human errors in the calculation and reporting of the measurements taken, and to practical problems arising from the large number of manual operations heretofore required in data acquisition and processing. These prior-art measuring systems have failed to measure some of the key parameters affecting the determination of the leakage rate, including volume, inclination, and the temperatures in the tank and in the piping. These factors have resulted in many test results showing what is known as a false negative, that is, test results indicating that there was no leak when actually leakage was occurring. They also resulted in what is known as a false positive, in which the test results indicated that there was a leak, when in fact there was no leak. Moreover, the prior-art tests have been unable to discriminate reliably between tank and piping leaks.
Moreover, test methods presently in common use, have not been capable of reliably and consistently measuring the leakage rate to the accuracy required by various government agencies. For example, California and some other states require an accuracy to within 0.05 gallons per hour of leakage in the test. These accuracies have not been possible with most of the methods and equipment heretofore in use. Moreover, many of the people involved in taking the needed measurements, have not been properly aware of the many ways in which errors can creep in.
Claims of specified degrees of accuracy for these prior-art test methods have been incorrect, being off by factors ranging up to ten times and even more, and can be shown to be exaggerated, using an error analysis that covers all significant factors affecting the accuracy of these methods.
The traditional and the most commonly used volumetric tank leak measurement methods have been very time consuming. Even when they have been properly employed, such methods have required an excessive number of hours for completion of a proper volumetric tank leak test. The long test times involved have resulted in high costs because of lengthy test-conduction time and also to the down time of the operation involved while the tank is not in operation.
As examples of the several types of systems that have been used to make these tests heretofore, two commonly used methods will be described.
(1) A first prior art method is known as a fluid static test. The tank and a standpipe are filled with liquid, and a loss or gain in volume in the tank and in the fill pipe is then measured by using a one-gallon graduated column in the standpipe. A single temperature sensor has been installed in the tank for manual measurement of the temperature of the liquid, apparently relying on a mistaken assumption that the temperature is the same throughout the liquid. An attempt has been made to obtain uniformity of liquid temperature by installing a circulating pump and a tank discharge hose, but is not sufficient to achieve a uniform liquid temperature. In this system manual measurements have also been made by visual readings of the change in liquid level in the graduated column and a hydrometer for specific gravity of the liquid. From these few measurements a liquid leakage rate for the tank has been calculated.
This first method is unable to discriminate between tank piping leaks and tank leaks, because the standpipe must be used above the level of the liquid in both the tank and the piping. Inaccuracies here could lead to unnecessary excavations and tank removal. Moreover, this first method is unable to take into account liquid level changes due to external tank piping which is not part of the fill pipe. Furthermore, this first method has required manual recording of data, visual measurement of the liquid level changes, and manual computation of the leakage rates, all of which tend to give rise to error. Further, the manual computation of the leakage rate does not take into account all the factors of significance to the accurate determination of the leakage rate. Hence, the accuracy of the system has been very limited, while the time needed to conduct the tests has been excessively long.
(2) A second prior art method has been operated on the principle that an apparent loss in weight of any object submerged in the liquid equals the weight of the displacement volume of the liquid. This method therefore employs a hollow cylinder which is sealed at its bottom end, an analytical balance, and a strip chart recorder. The analytical balance includes a sensor suspended from it into the tank liquid. The weight of the sensor, as measured by the analytical balance, is equal to its actual weight minus the buoyancy force from the liquid in the tank. Changes in liquid level from a leak or from other causes will change the buoyancy force on the hollow cylinder and the weight of the sensor, and the analytical balance measures the weight change. The analytical balance has then converted this weight change to an electrical signal transmitted to the strip chart recorder, which shows volume change plotted against time. The angle and length of the line drawn by the recorder has been directly related to the quantity and rate of the leakage. Meanwhile, a thermistor has been lowered into the center of the tank, and the temperature of the liquid at that single location measured. Volumetric changes in the liquid are determined by manually computing only the product of the temperature change, the volume of the tank, and the coefficient of expansion of the liquid in the tank. Therefore, the system accuracy and reliability of this second method are also quite limited. This method does not account for many sources of possible error, including the possible effects of ground water on the tank and on the leakage rate therefrom. The single temperature sensor in the tank does not accurately measure the volumetric changes due to temperature, because there are different temperatures in the tank, with a temperature gradient from the top all the way to the bottom of the tank. The manual calculations and recordings are also subject to human error.
Other prior-art systems have severe accuracy limitations, or do not account for or measure key factors that affect the leakage rate of the tank, or have prior resolution of the leak rate, or all or several of these problems. They tend to result in "false negatives" and "false positives". All of them require long test times.
Without further reviewing other systems, it can generally be stated that there are many causes of inaccuracy in these and other methods. These causes vary in either positive or negative amounts from a zero reference point. These causes of error may be listed as follows:
1. The liquid is subject to thermal expansion and contraction, which affects density considerations as well as the actual volume to be measured.
2. There is temperature stratification of the fluid contained in the storage tank; therefore temperature measurements at any one level do not give a proper reflection of the various temperatures of the liquid in the tank and its associated piping or the average temperature, and the effects of these temperatures.
3. Evaporation of the liquid from the tank and its piping during the test may appear to be leakage and may be treated as leakage, because any reduction in volume is assumed to be leakage. Evaporation not compensated for is usually recorded as leakage, even when in fact there is no leakage whatever.
4. Vapor pockets are present in many tanks and in the associated piping during the measurement, and these are not accounted for.
5. There are pressure variations in the liquid and also in the atmospheric pressure above the liquid in the storage tank. Since tests take a significant time to conduct, there is bound to be error from this source, unless these pressure variations can in some way be taken account of and included in the calculations. Current methods have not been able to take these pressure variations into account.
6. The ends of the storage tanks deflect during and after filling. This deflection results in an increase in the tank volume. The rate of volume increases with time until stability is reached, and then there is no additional change in tank volume due to deflection. The increased volume in the tank lowers the load of the liquid in the tank and gives the appearance of a leak.
7. The water table and effects of ground water have not been considered, although their effects may be a significant source of error in the final results. For example, if the water table is above any portion of the tank, a leak could be missed, due to the higher pressure head.
8. The geometry of the tank under consideration has usually been neglected, it being assumed that so long as the tank has a certain volume under certain conditions, the geometry of the tank is not important to consider. However, this geometry may seriously affect the accuracy of the final results. Many systems are sensitive to the product level and/or to the temperature, so that tank and piping specifications effect the accuracy of leakage measurements. The differences between the manufacturer's specifications and the actual tanks and piping are important. For example, measurement of the liquid level in piping demands on accurately knowing the internal diameter of the piping, the cross-sectional area of the instrumentation inside the piping, and the cross-sectional area of other attached piping. Volumetric change due to temperature depends on the actual volume of the tank and piping, not the nominal specifications. The level measurement in the tank depends on the cross-sectional area of the tank in the vertical axis.
9. The effect of wind has almost never been considered. Yet wind can effect the level measurements, the temperature, the pressure, and the evaporation rate of the liquid. For example, a strong wind can create a wave in the exposed piping, resulting in irregular fluctuations in the liquid level.
10. Vibration is present in some tanks but not in others. Vibration may be due to wind, traffic, earthquakes, or construction work. It may vary due to the nearness to a highway used by large trucks, sometimes during measurements and sometimes not. Whatever the cause, vibration of the tank is a serious cause of error. Vibration effects increase with the free surface area under test and is much larger when measuring in the tank instead of in the fill pipe. Vibration can also affect the temperature measurement by moving the temperature sensor and causing erros in its reading. This is a particular problem in large tanks, such as a 50,000-gallon tank, where temperature measurement accuracies of better than 0.003.degree. F. are necessary.
11. Noises from acoustical sounds and electrical noise are other sources of error that have generally been disregarded when trying to detect tank leaks.
12. The accuracy of the equipment is a well-known source of possible error, yet most of the equipment used simply does not have the accuracy required. The instruments must be precise in order to avoid error, because if the instruments themselves are inaccurate, the results will be; if several measurements are taken and with different degrees of accuracy, one cannot know how much inaccuracy there is, or whether the inaccuracies balance or add to each other.
13. The limitations of the instrumentation have generally not been considered, although they are quite important. Typical limitations include thermocouple accuracy, the inability of certain equipment to make accurate measurements because of piping inclinations or even to measure in piping at all. Some instruments cannot give correct values in small pipes, including the fill pipe; some systems are unable to distinguish between tank leaks and piping leaks; some cannot operate accurately in some types of liquids; some cannot take groundwater into account. None can operate accurately without removing the tank's drop tube.
14. The type of liquid in the tank and its specific gravity is often not taken into account and can produce error.
15. Operator error is one of the most significant sources of error and has generally been underestimated or assumed not to be present. It is present to some extent in all tests, but it is most prevalent and significant in systems that require manual measurements and manual computations.
16. Power variations occur, and equipment responsive to these power variations is therefore subject to the errors introduced by these variations. Rarely has compensation been made for this or steps taken to eliminate such power variations.
17. Atmospheric pressure has, in most instances, not been measured at all, and it has generally been assumed to remain constant during the test. It may be that in some tests the atmospheric pressure does not change and that a standard measurement is sufficient, but usually this is not the case, because altitude and climatic changes, as well as weather, affect the atmospheric pressure, which should therefore always be directly considered. Atmospheric pressure changes are particularly significant in cases where there are vapor products in the tank or piping.
18. Inclination of the tank fill pipe and of the tank has generally been disregarded. The assumption is made, without even considering it, that there is no such inclination, yet there usually is, and that inclination can affect the accuracy of measurement. In an inclined pipe or tank, the volume change per unit level change is different from that of horizontal pipes and tanks.
19. The tank pressure during the test may exceed the normal operating pressure. When this happens, and when proper allowances are not made, errors inevitably occur. Higher tank pressure results in a higher leakage rate than for normal tank pressure. Also, high tank pressures can damage tanks.
20. Prior-art measuring systems have usually not discriminated between piping leaks and tank leaks. Discrimination is extremely important. If the tank leaks, tank replacement is normally required. When the leak is in the piping, a completely different remedy is, of course, required. Hence, if the results of the test are wrong, high and unnecessary costs result.
21. When the liquid level is below the level of the fill pipe, the measurement of tank leakage becomes extremely difficult and requires a system that is 1000 to 10,000 times more accurate. No previous system has been capable of measuring leakage rates in the tank, rather than in the fill pipe.
22. The level of the product in the tank affects the leakage rate. If the liquid level is higher than the normal maximum level, the leakage rate is higher than normal. The leakage rate is proportional to the square root of the pressure. Also, a lower-than normal pressure results in a lower-than-normal leakage rate.
23. The leak rate may vary, depending on various factors which have usually not been considered, such as the location of the tank in relation to the liquid level in the tank and piping, the type of liquid in the tank and the groundwater level.
24. Differential pressures should be considered for methods employing either pressure or level measurements in computation of the product level variations, which are due to pressure, temperature, and leakage, but generally have not been.
25. Hydrostatic pressure and the properties of the liquid, such as its bulk modulus, should always be considered, but generally have not been.
26. The true volume of the tank is important, and yet has usually not been considered. It has been assumed to be whatever its nominal volue is, rather than its actual value. Any difference from true volume reflects directly the leakage rate error and introduces an error into the calculations.
27. Unusual events may affect the accuracy of the measurement. These may include unexpected movement of the measuring instruments during the tests and so-called "acts of God", etc.
28. A volumetric measurement error is often obtained due to the effect of the volume of the liquid in the vent pipes, the vapor recovery lines, and the fuel lines, in relation to the volume of the liquid in the fill pipe riser. Usually, nothing has been done to take this source of error into account.
29. There are temperature effects traceable to the liquid in the piping that is connected to the tank, but these have usually been disregarded, and another source of error thereby has entered.
30. When the volume in the tank is unknown, there is also a temperature effect that cannot properly be considered. This, also, gives rise to errors.
31. The piping also may have an unknown volume or one that is not being considered, and this may affect the accuracy of the temperature measurement and therefore the final results.
32. There may be a volumetric measurement error, if the true volume of the tank is not known.
33. Often the temperature coefficient of expansion of the liquid is not determined precisely. For example, the coefficients of expansion of various types of gasoline--leaded, unleaded, "regular", "aviation", jet fuel, for example--are not normally determined in current test methods or taken into account, and this neglect affects the accuracy of the results.
34. When the partly underground tank is being filled from a tank truck, the temperature of the two liquids may be at greatly different temperatures, and failure to take both temperatures into account can lead to substantial errors.
35. Water present in the storage tank has a different coefficient of expansion for gasoline or oil, and treating them as identical leads to errors of computation.
A few systems have proposed to take some of these errors into account, but all systems, of which I am presently aware, have not considered many of these limitations on their accuracy. No one method currently in use is able to solve a significant number of the thirty-three problems listed above. Heretofore, no one method has solved even a significant number of these problems.
Systems currently in use have also required long test times to provide even a degree of optimization of their rather poor accuracy measurements.